For her birthday, Ruthie got 4 math books and some other books, all different. From these, she chooses 2 math books and one other book to take on vacation. There are 30 different sets of books Ruthie could choose. How many books in all did she get for her birthday?
number of choices of math books = C(4,2) = 6
number of choices of one of x others = C(x,1) = x
6x = 30
x = 5
So she got 4 math books and 5 others, or 9 in total